Powder coatings, which are dry, finely divided, free flowing, solid materials at room temperature, have gained considerable popularity in recent years over liquid coatings for a number of reasons. For one, powder coatings are user and environmentally friendly materials, since they are virtually free of harmful fugitive organic solvent carriers that are normally present in liquid coatings. Powder coatings, therefore, give off little, if any, volatile materials to the environment when cured. This eliminates the solvent emission problems associated with liquid coatings, such as air pollution and dangers to the health of workers employed in coating operations.
Powder coatings are also clean and convenient to use. They are applied in a clean manner over the substrate, since they are in dry, solid form. The powders are easily swept up in the event of a spill and do not require special cleaning and spill containment supplies, as do liquid coatings. Working hygiene is, thus, improved. No messy liquids are used that adhere to worker's clothes and to the coating equipment, which leads to increased machine downtime and clean up costs.
Powder coatings are essentially 100% recyclable. Over sprayed powders can be fully reclaimed and recombined with the powder feed. This provides very high coating efficiencies and also substantially reduces the amount of waste generated. Recycling of liquid coatings during application is not done, which leads to increased waste and hazardous waste disposal costs.
In the past, most powder coating was performed on metals which can withstand high temperatures at which many conventional coating powders fuse and cure. Recently, however, several coating powders have been developed for substrates, such as wood, fiberboard, certain plastics, etc., which require coating powders which fuse (in the case of thermoplastic coating powders) or fuse and cure (in the case of curable coating powders) at relatively low temperatures. Examples of such coating powders are found, for example, in U.S. Pat. Nos. 5,824,373, 5,714,206, 5,721,052, and 5,731,043, the teachings of each of which are incorporated herein by reference. Low temperature coating prevents charring of the substrate and helps to prevent excessive outgassing of moisture.
A frequent problem encountered when coating low-temperature substrates, such as wood, with coating powder is non-uniformity of coating in areas of the substrate which are difficult to coat, such as the edges and corners of kitchen cabinet doors. A frequently observed defect at edges, corners and other surface discontinuities is cracking. While applicants are not bound by theory, such cracking at edges, corners, etc. may be the result of differential thermal expansion and contraction of the substrate and the fusing (in the case of thermoplastic coating powders) and fusing and curing (in the case of curable coating powders) coatings. Coating powders for heat-sensitive substrates, such as wood, are typically fused and cured at temperatures between about 200 and about 350.degree. F. (between about 93 and about 177.degree. C.); and coating powders for other substrates at temperatures up to about 450.degree. F. (449.degree. C.). Coatings produced from coating powders are further vulnerable to cracking at edges and corners because such coatings are relatively thick, i.e., typically being between about 3 and about 10 mils thick (75 to 250 microns).
Accordingly, it is a general object of the present invention to reduce cracking of coatings, derived from coating powders, particularly at surface discontinuities.
Cellulosic substrates, such as wood, fiberboard, etc. generally contain some moisture, e.g., between about 3 and about 10 wt %. This moisture is advantageous for electrostatic application of coating powder in that it enables the otherwise non-conductive material to hold sufficient electrical charge for efficient electrostatic coating powder application. However, the moisture is also disadvantageous in that outgassing of moisture, as well as outgassing of other volatiles, during fusing or fusing/curing, can lead to defects such as pinholes or blisters in the coating. In a wood substrate of generally uniform density and composition, outgassing may be relatively evenly distributed throughout the surfaces and edges. In medium- to high-density fiberboard which is denser in surface regions than interior regions, outgassing is particularly problematic at the machined edges because the less dense interior core region provides a lateral pathway for outgassing volatiles.
Accordingly, it is further an object of the invention to reduce defects in substrates containing water and other volatile chemicals which may outgas during fusing or fusing/curing of the coating powder.